formation of ag nanoparticles and enhancement of tb3+ luminescence in tb and ag co-doped...

Formation of Ag nanoparticles and enhancement of Tb3+ luminescence in Tb and Ag co-doped lithium-lanthanum-aluminosilicate glass

Patryk Piasecki Ashley Piasecki Zhengda Pan Richard Mu Steven H. Morgan

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Formation of Ag nanoparticles and enhancement of Tb3+ luminescence in Tb and Ag co-doped lithium-

lanthanum-aluminosilicate glass

Patryk Piasecki, Ashley Piasecki, Zhengda Pan,* Richard Mu, and Steven H. Morgan

Fisk University, Department of Physics, 1000 - 17th Avenue North Nashville, TN 37208 USA

*[email protected]

Abstract. Tb3+ and Ag co-doped glass nano-composites were synthesized in a glass matrix Li2O-LaF3-Al2O3-SiO2 (LLAS) by a melt-quench technique. The growth of Ag nanoparticles (NPs) was controlled by a thermal annealing process. A broad absorption band peaking at about 420 nm was observed due to surface plasmon resonance (SPR) of Ag NPs. The intensity of this band grows with increasing annealing time. The transmission electron microscopic image (TEM) reveals the formation of Ag NPs in glass matrix. Photoluminescence (PL) emission and excitation spectra were measured for glass samples with different Ag concentrations and different annealing times. Plasmon enhanced Tb3+ luminescence was observed at certain excitation wavelength regions. Luminescence quenching was also observed for samples with high Ag concentration and longer annealing time. Our luminescence results suggest that there are two competitive effects, enhancement and quenching, acting on Tb3+ luminescence in the presence of Ag NPs. The enhancement of Tb3+ luminescence is mainly attributed to local field effects due to SPR. The quenching of luminescence suggests an energy transfer from Tb3+ ions to Ag NPs.

Keywords: Ag, nanoparticle, SPR, Tb3+, luminescence, glass.

1 INTRODUCTION Rare-earth doped glasses are important for applications in optical devices such as scintillators, optical fiber amplifiers, lasers, optical converters of infrared radiation to visible regions, etc [1-4]. In addition to the common advantages of low-cost, large-volume production possibility and easy shaping of elements of glasses, oxide glasses generally possess good mechanical strength, chemical durability, and thermal stability. Tb-doped Li2O-LaF3-Al2O3-SiO2 (LLAS:Tb) glass was previously reported to have good photoluminescence (PL) and β-induced luminescence light-yield for scintillator application [1].

The incorporation of metallic nanoparticles (NPs) in rare-earth doped glasses may enhance the luminescence of rare-earth ions when the excitation or emission is near the SPR wavelength of metallic NPs [5-8]. Coupling rare-earth ions with metallic NPs has been developed as a valuable strategy to improve the luminescence yield of rare-earth ions [7,9,10]. A basic interest in these composite materials is to know under what conditions the emission yield will increase or decrease. The enhancement has been attributed to local field enhancement effects and energy transfer from metallic NPs to rare earth ions [9]. The SPR of Ag NPs causes an intensified local electromagnetic field around the NPs, resulting in enhanced optical transitions of rare-earth ions in the vicinity. The mechanism of energy transfer from metallic NPs to rare earth ions has not been clarified [5,6]. However, the energy transfer from ions to the metallic NPs may quench rare-earth luminescence, if this is not compensated by the local field enhancement effects [5,6].

Journal of Nanophotonics, Vol. 4, 043522 (1 December 2010)

© 2010 Society of Photo-Optical Instrumentation Engineers [DOI: 10.1117/1.3528943]Received 11 Oct 2010; accepted 22 Nov 2010; published 1 Dec 2010 [CCC: 19342608/2010/$25.00]Journal of Nanophotonics, Vol. 4, 043522 (2010) Page 1


In order to further understand the interaction between metallic NPs and rare earth ions in glass, we have studied Tb-doped LLAS glass containing Ag NPs. In present work, we report optical absorption, TEM, photoluminescence emission and excitation of Ag co-doped LLAS:Tb glass with different Ag concentrations and annealing times, and discuss the possible mechanisms for plasmonic enhancement and quenching effects on Tb3+ luminescence.

2 EXPERIMENT Reagent grade anhydrous oxide powders of Li2CO3, LaF3, Al2O3, SiO2, TbF3, and AgNO3 were used to prepare glasses. The compositions of lithium-lanthanum-aluminosilicate glass were 28Li2O-11LaF2-6Al2O3-55SiO2 (LLAS). Four samples containing the same TbF3 concentration of 1.0 mol % and four different AgNO3 concentrations of 0.0 mol %, 0.1 mol %, 0.2 mol %, and 0.5 mol %, were prepared. Batches of 30 grams were thoroughly mixed in an agate mortar, and further mixed in a Ball Mill for four hours. The mixture was melted in a platinum crucible at a temperature of 1430 °C. The melts were held for 50 minutes and then cast onto a copper plate and pressed by another copper plate from the top, forming a glass disk of about 4 mm in thickness. The glasses were subsequently annealed at 330 ºC for 15 minutes and then allowed to cool to room temperature in the furnace. Clear glasses were formed for all batches with different Ag concentrations. These glasses appear to be of very good optical quality, with no visual evidence of devitrification. Further thermal annealing was performed at 480 ºC in air.

A Philips EM420 TEM operated at 120KV in bright field modes was used to observe the NPs. The short beam exposure was to reduce any avoid beam damage. The samples were cleaved from the bulk samples by gently depressing on the edge of the sample with a clean razor blade, to cleave a small sliver from the sample. After cleaving, the sample sliver was immediately collected onto a 200 mesh grid and examined in the TEM. UV-visible absorption was measured from 300 to 600 nm using a Cary dual beam spectrophotometer. PL was measured using 325 nm laser excitation. The excitation spectra were taken using HORIBA FL3-11 Spectrofluorometer with a xenon lamp.

3 RESULTS

3.1 Optical absorption and TEM Figure 1 shows the optical absorption spectra of three samples doped with 0.1 mol %, 0.2 mol %, and 0.5 mol % Ag. The spectra were taken for increasing annealing time intervals in hours (h). The annealing was performed at 480 ºC in air atmosphere. Absorption peaks from Tb3+ ions were difficult to observe because of the weak intra-4f transition in nature. A strong and broad absorption band (spreading from 320 to 520 nm) peaking at 420 nm was dominantly observed. This absorption band is the characteristic of SPR of Ag NPs in glass [7,8,11]. The amplitude of this absorption band increases for increasing annealing time. The increase of absorbance is significantly faster for sample with a higher Ag concentration. The peak absorbance reaches 4. 0 in 48 h annealing for sample with 0.1 mol % Ag and in less than 4 h for sample with 0.5 mol % Ag. It is generally believed that thermal annealing causes reduction of Ag ions and growth of Ag NPs, resulting in increase of the Ag NPs concentration and size in the glass matrix [7,8]. The metal NP growth is associated with the reduction of the metal colloid [10,12]. The observed increase of absorbance due to SPR of Ag NPs is therefore attributed to the growth of Ag NPs in the glass matrix [11].

Figure 2 shows the peak absorbance as a function of annealing time for three samples doped with 0.1 mol %, 0.2 mol %, and 0.5 mol % Ag. The data are well fitted with second-order polynomials. The slope of the curve represents the rate of absorbance increase with annealing time. The rate is largest for the sample with 0.5 mol % Ag and smallest for the

Journal of Nanophotonics, Vol. 4, 043522 (2010) Page 2


sample with 0.1 mol % Ag. This result suggests that the growth of Ag NPs is faster for samples with a high Ag concentration. Fig. 1. Optical absorption spectra of Ag co-doped LLAS:Tb glass: (a) 0.1 mol % Ag, (b) 0.2 mol % Ag, and (c) 0.5 mol % Ag. Annealing is at 480 ºC in air. The SPR absorption band centered at 420 nm grows up with increasing annealing time. Fig. 2. Peak absorbance due to SPR of Ag NPs versus annealing time: (a) 0.1 mol % Ag, (b) 0.2 mol % Ag, and (c) 0.5 mol % Ag. The data are fitted with second-order polynomials.

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Figure 3 shows a typical TEM image of the sample co-doped with 0.5 mol % Ag and annealed at 480 ºC for 3 h, demonstrating the presence of near-spherical NPs in glass matrix. The image is representative and should represent the particle density and size of the Ag particles. The corresponding size distribution histograms are presented in fig. 3 (b). The mean diameter of particles is 5.6 nm. The average size of NPs was found to increase with increasing Ag concentration and annealing time. No nanoparticles can be observed from as-prepared glass samples. (a) (b) 20 nm Fig. 3. TEM results for sample co-doped with 0.5 mol % Ag and annealed at 480 ºC for 3 h: (a) a typical TEM image, (b) particle size distribution of the Ag NPs.

3.2 Plasmon enhanced Tb3+ luminescence excited at 325 nm The luminescence spectra were measured for samples with three different Ag concentrations, 0.1 mol %, 0.2 mol %, 0.5 mol % and for different annealing times, compared to a reference sample without Ag co-doping. The excitation source used was a cw HeCd laser at 325 nm. All measurements were performed under the same experimental conditions with the same sampled volume for comparison. Figure 4 shows the selected luminescence spectra from three samples with different Ag concentrations and annealing times: (a) PL from the sample with 0.1 mol % Ag and 24 h annealing, (b) PL from the sample with 0.2 mol % Ag and 9 h annealing, and (c) PL from the sample with 0.5 mol % Ag and 3 h annealing, all compared with a spectrum from the reference sample. Four major emission bands of Tb3+ at 489, 542, 585, and 622 nm were observed. These emission peaks are attributed to transitions of 5D4 → 7Fi (i = 6, 5, 4, and 3, respectively) [1]. PL results in Fig. 4 illuminated the enhancement of Tb3+ luminescence in the presence of Ag NPs in glass matrix. The PL enhancement factor is 1.2, 1.9, and 2.9 for sample (a), (b), and (c), respectively. The highest PL intensity is from the sample with 0.5 mol % Ag and three h annealing time. In addition to the enhanced Tb3+ luminescence, background emission between 450 to 650 nm was observed from samples (b) and (c). This background emission increases with Ag concentration, and is therefore assumed to be from Ag related defects in the glass.

Figure 5 shows PL enhancement factor versus different annealing times for the sample with 0.5 mol % Ag. The largest value is 2.84 for 3 h annealing and the smallest is 0.88 for 8 h annealing. The reduced PL intensity suggests a quenching effect on Tb3+ luminescence from the sample with a higher Ag concentration and longer annealing time.

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Fig. 4. Plasmon enhanced Tb3+ luminescence in Ag co-doped LLAS:Tb glass: (a) 0.1 mol % Ag, (b) 0.2 mol % Ag, and (c) 0.5 mol % Ag, compared with a reference sample without Ag co-doping. Fig. 5. PL enhancement factor versus different annealing times for a glass sample with 0.5 mol % Ag (for PL peak at 542 nm and using excitation at 325 nm).

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3.3 Excitation spectra Figure 6 shows the excitation spectra for luminescence at 542 nm (5D4 → 7F5) as the excitation wavelength tuning from 300 to 538 nm, where the spectrum (a) is from the sample with 0.1 mol % Ag and 24 h annealing, (b) is from the sample with 0.2 mol % Ag and 9 h annealing, and (c) is from the sample with 0.5 mol % Ag and 3 h annealing, each spectrum is compared to the spectrum from the reference sample without Ag. Excitation spectra demonstrate the excitation wavelength dependence of Tb3+ luminescence. The relative intensity from samples with Ag NPs compared to that from the reference sample reflects the combined effects of the enhancement and quenching on Tb3+ luminescence. Multiple excitation peaks were observed at 317, 339, 351, 368, 377, and 484 nm, which correspond to the absorption peaks of Tb3+ ions. Excitation spectra in Fig. 6 indicate that the luminescence of Tb3+ ions may increase or decrease in the presence of Ag NPs. This change in intensity is dependent on the excitation wavelength, Ag concentration, and annealing time.

Fig. 6. Excitation spectra of Ag co-doped LLAS:Tb samples: (a) 0.1 mol % Ag, 24 h, (b) 0.2 mol % Ag, 9h, and (c) 0.5 mol % Ag, 3 h, compared with a reference sample without Ag co-doping (for emission 5D4 → 7F5 at 542 nm).

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Excitation spectrum (a) indicates that the Tb3+ luminescence increases at the excitation wavelength between 345 to 380 nm and near 484 nm, including four resonant excitation wavelengths of 350, 367, 376, and 484 nm. Excitation spectrum (b) indicates that the Tb3+ luminescence increases near 325 nm, which is a non-resonant wavelength, but decreases between 345 to 385 nm. Excitation spectrum (c) indicates that the Tb3+ luminescence increases more significantly near 325 nm (a non-resonant wavelength), but decreases significantly between 345 to 385 nm, and near 484 nm. For all spectra, no significant Tb3+ luminescence was observed for the excitation wavelength near 420 nm, which is the peak of SPR of Ag NPs.

4 DISCUSSION The enhancement of rare earth luminescence influenced by SPR of metallic NPs is generally attributed to two mechanisms: the local field enhancement (LFE) and the energy transfer from metallic NPs to rare-earth ions [9]. The effects of energy transfer between ions and particles are twofold: they may enhance luminescence by energy transfer from particles to ions and/or may quench luminescence by energy transfer from ions to particles [8-11,13]. In our case, the light at 420 nm will certainly excite the SP of Ag NPs, but no significant Tb3+ luminescence was observed [Fig. 6]. This result does not support the energy transfer from Ag NPs to Tb3+ ions and is consistent with a previous argument proposed by O. L. Malta, that energy transfer from the particles to the ions is not expected to be operative since the plasma lifetime is extremely short [6]. The observed enhancement of Tb3+ luminescence may be attributed to LFE due to SPR of metallic NPs. According to LFE, there is an additional interaction between electromagnetic field and rare-earth ions due to the very high field gradients near a metallic particle [5,6]. The local field effects may enhance absorption, emission, and energy transfer from glass host to Tb3+ ions [5, 6]. The expressions for Lorentz local-field correction for the various mechanisms of absorption and emission of light are │Eeff/E0│2 n3 for absorption and induced emission and │Eeff/E0│2 n for spontaneous emission, where E0, Eeff

, and n are the incident field, the local effective field in the medium and the index of refraction of the medium, respectively [5,7]. In addition, different Tb3+ ions are under different local field strength due to their different distance to Ag NPs, the local field could influence the Stark-splitting of Tb3+ energy levels and increase the inhomogeneous broadening of the excitation of Tb3+ ions in glass [3]. The increased inhomogeneous broadening could result in the observed spreading of the excitation peaks, particularly near 325 nm as shown in 6 (b) and (c). The enhanced luminescence for excitation at 325 nm could be attributed to the increased absorption and energy transfer from glass host to Tb3+ ions in the presence of Ag NPs. According to previous discussion, the excitation and emission wavelength, host matrix, binding state, size and shape of Ag NPs, and distance between the rare-earth ions and NPs are all important factors in LFE [5-11,13]. The decrease of luminescence was observed for samples containing Ag NPs with higher Ag concentration of 0.2 mol % and 0.5 mol %, particularly in the excitation wavelength between 345 to 380 nm, including three resonance wavelengths of 350, 367, and 376 nm. This result indicates that quenching exceeds enhancement in this region for samples (b) and (c). The quenching effect is attributed to the energy transfer from excited Tb3+ ions to Ag NPs [5,6,8,13]. The observed Tb3+ luminescence quenching is more severe for the sample with 0.5 mol % Ag and longer annealing time, which corresponds to a high particle volume fraction and large particle size of Ag NPs, consistent with previous reported results [8-11,13].

The observed enhancement and quenching of Tb3+ luminescence depends on excitation wavelength, and this dependence is further related to Ag concentration and annealing time. For sample (a) (0.1 mol % Ag, 24 h), the enhancement of PL is more significant in resonance wavelength region and closer to the peak of SPR. However, for sample (c) (0.5 mol % Ag, 3h), the enhancement of PL is observed near 325 nm, which is a non-resonant wavelength of



Tb3+ and relatively away from SPR peak of Ag NPs, but the quenching of PL is more severe from 345 to 385 nm, and near 484 nm. It is likely that the quenching effect increases rapidly when Ag particle volume fraction or size surpassed a certain point. The exact mechanism has not yet been fully understood.

5 CONCLUSION Tb-doped Li2O-LaF3-Al2O3-SiO2 (LLAS:Tb) glasses containing different Ag concentration and annealed at different time intervals were studied. LLAS glass matrix provides a convenient host for Tb doping and for producing Ag NPs revealed by TEM image. A strong and broad absorption band centered at 420 nm due to SPR of Ag NPs has been observed. The amplitude of this SPR absorption band increases systematically with annealing times. The enhancement of Tb3+ luminescence was observed for glass containing Ag NPs at certain excitation wavelength regions, dependent on the Ag doping concentration and annealing time. The luminescence enhancement in the presence of Ag NPs is attributed to the local field effects due to SPR of Ag NPs. Ag NPs in the glass may also participate in an observed luminescence quenching for excitation wavelengths from 345 to 380 nm and 484 nm, particularly at four resonance wavelengths of Tb3+ ions. This luminescence quenching is attributed to an energy transfer process from excited Tb3+ ions to Ag NPs. The energy transfer from Tb3+ ions to Ag NPs provides a path for extra non-radiative loss of excited Tb3+ ions. This luminescence quenching is more significant for samples with higher Ag concentrations and longer annealing times.

Acknowledgments This research is supported by US National Science Foundation NSF-CREST- CA: HRD-0420516, NSF-STC CLiPS - grant no. 0423914, and NSF CBET-0829977.

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Morgan, "Terbium-activated lithium-lanthanum-aluminosilicate oxyfluoride scintillating glass and glass-ceramic," Nucl. Instr. Meth. A594, 215-219 (2008) [doi:10.1016/j.nima.2008.06.041].

[2] Z. Pan, A. Ueda, R. Mu, and S. H. Morgan, "Upconversion luminescence in Er3+ -doped germinate-oxyfluoride and tellurium-germanate-oxyfluoride transparent glass-ceramics," J. Lumin. 126, 251-256 (2007) [doi:10.1016/j.jlumin.2006.07.021].

[3] Z. Pan, A. Ueda, S. H. Morgan, and R. Mu, "Luminescence of Er3+ in oxyfluoride transparent glass-ceramics," J. Rare Earths 24, 699-705 (2006) [doi:10.1016/S1002-0721(07)60012-X].

[4] Z. Pan, A. Ueda, S. M. Hays, R. Mu, and S. H. Morgan, "Studies of Er3+ doped germinate-oxyfluoride and tellurium-germanate-oxyfluoride transparent glass-ceramics," J. Non-Cryst. Solids 352, 801-806 (2006) [doi:10.1016/j.jnoncrysol.2006.01.023].

[5] O. L. Malta, P. A. Santa-Cruz, G. F. De Sá, and F. Auzel, "Fluorescence enhancement induced by the presence of small silver particles in Eu3+ doped materials," J. Lumin. 33, 261-272 (1985) [doi:10.1016/0022-2313(85)90003-1].

[6] O. L. Malta and M. A. Couto dos Santos, "Theoretical analysis of the fluorescence yield of rare earth ions in glasses contaning small metallic particles," Chem. Phys. Lett. 174, 13-18 (1990) [doi:10.1016/0009-2614(90)85319-8].

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[10] T. Som and B. Karmakar, "Nanosilver enhanced upconversion fluorescence of erbium ions in Er3+: Ag-antimony glass nanocomposites," J. Appl. Phys. 105, 013102 (2009) [doi:10.1063/1.3054918].

[11] S. De Marchi, G. Mattei, P. Mazzoldi, C. Sada, and A. Miotello, "Two stages in the kinetics of gold cluster growth in ion-implanted silica during isothermal annealing in oxidizing atmosphere," J. Appl. Phys. 92, 4249-4254 (2002) [doi:10.1063/1.1506423].

[12] G. Mpourmpakis and D. G. Vlachos, "Growth mechanisms of metal nanoparticles via first principles," Phys. Rev. Lett. 102, 15505 (2009) [doi:10.1103/PhysRevLett.102.155505].

[13] J. A. Jiménez, S. Lysenko, and H. Liu, "Enhanced UV-excited luminescence of europium ions in silver/tin-doped glass," J. Lumin. 128, 831-833 (2007) [doi:10.1016/j.jlumin.2007.11.018].

Patryk Piasecki graduated in May, 2009 with a B.S. in Physics from Austin Peay State University in Clarksville, TN. He is currently a graduate physics research assistant at Fisk University in Nashville, TN working on his Master’s Thesis. Ashley Piasecki graduated in December, 2009 with a B.A. in English from Austin Peay State University in Clarksville, TN. She is currently a graduate physics research assistant at Fisk University in Nashville, TN working on her Master’s Thesis. Zhengda Pan is a research associate professor of Physics at Fisk University in Nashville, TN, USA. He has a Ph.D in Physics and is the author of more than 50 journal papers and 5 US patents. His research interests include laser spectroscopy of solids and optical materials for photonic applications. He is a member of OSA and MRS. Richard Mu is a professor of Physics at Fisk University in Nashville, TN, USA. He has a Ph.D in Physics. Steven H. Morgan is a professor and head of Physics Department at Fisk University in Nashville, TN, USA. He has a Ph.D in Physics.



formation of ag nanoparticles and enhancement of tb3+ luminescence in tb and ag co-doped...

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